This invention relates to rear projection screens, and more particularly relates to such screens for use projectors in which the projection beam is nearly coherent.
Rear projection screens transmit to an audience space an image projected onto the rear of the screen. The performance of rear projection screens is characterized by their gain (defined as the luminance of the screen in the forward going direction with respect to the luminance from an ideal lambertian reflector), their viewing space, resolution, contrast and artifacts. An ideal rear projection screen would provide a clear crisp, high contrast, bright image for a large audience space. That is, the screen would have 1) high resolution, 2) freedom from artifacts, 3) contrast enhancement, 4) high gain and 5) spreading of the light from the projector into a large range of vertical and horizontal angles.
In reality, a screen that exhibits all these characteristics does not exist. For instance, to increase screen gain, the screen designer must limit the audience space. Typically, viewers of rear projection displays are scattered over a large range of horizontal angles (i.e. sitting anywhere in a room) but they all fall within a limited range of vertical angles (no one is near the ceiling or directly below the screen). Therefore to increase the brightness of the system, screen gain can be increased by limiting the vertical distribution of the light from .+-.90 degrees to typically .+-.8 degrees. By `squeezing` the light into this limited audience space, the gain of the screen can be made to be as high as 6.0.
High resolution requires very small structures in the screen. High contrast requires a mechanism to reduce the amount of ambient room light that is reflected from the screen. Typically, either a black dye is added to the screen or black stripes are added to the front surface of the screen to reduce reflections of ambient light. With black dye, the contrast is increased but the gain is reduced (light from the projector is absorbed along with the ambient light). With black stripes, the resolution is limited by the pitch of the stripes.
Therefore different screen design concepts must be used to design the optimal screen for a specific application.
The commercial market for rear projection screens can be divided into two main categories; screens for consumer rear projection TVs (PTV), and screens for specialty applications.
Almost all PTVs use double lenticular, high contrast, high gain screens. FIG. 1 shows in a longitudinal cross section a typical double lenticular screen, described for example in U.S. Pat. No. 5,066,099. The screen is made of two pieces. The rear piece is a Fresnel lens 10 which is generally designed to image the exit pupil of the projector to the viewing plane. This allows a viewer sitting on axis to see the entire image. The front piece 12 has front 14 and rear 16 vertically oriented lenticular surfaces (one with black stripes 18) with bulk diffusion between the two surfaces. A bulk diffuser is defined as minute colloidal particles 19 suspended throughout the screen. The particles (typically less than about 40 microns in size) have a slightly different refractive index than that of the screen. The bulk diffuser is designed to provide the desired vertical distribution of the image into the audience space, typically .+-.8 degrees.
The rear lenticular surface 16 focuses the light coming from the Fresnel lens into stripes at the plane of the front surface 14. The lenticules 14a of the front lenticular surface are aligned to these stripes of light and spread the light into a wide range of horizontal angles. In between these stripes, where the screen is not optically active, stripes 18 of black paint are applied. The black stripes do not effect the light coming out of the projector but do absorb about 50% of the ambient room light that hits the screen's front surface.
For this type of screen, there is a direct relationship between the pitch of the lenticular surfaces and the thickness of the screen. A minimum thickness is required to assure mechanical durability. This limits the minimum pitch (distance between the lenticules) to about 0.5 mm, which in turn limits the ultimate resolution of the screen.
Other screens are made for niche markets. Diffusing screens (rotationally symmetric bulk or surface diffusers) which have low gain (less than 2.0) and no contrast enhancement or dyes are used in high resolution systems such as microfilm/microfiche readers.
Non blackened lenticular screens (see FIG. 2) are typically one-piece. The Fresnel lens 20 is on the back surface and a lenticular surface 22 is on the front. There are diffusing particles 19 throughout the bulk. The combination of the bulk diffuser and lenticules provide a high aspect ratio viewing space and high gain. Since there is no relationship between the thickness of the screen and the pitch of the lenticules, resolution is limited only by the ability to manufacture the individual lenticular elements. The Fresnel can be put on the back surface when the distance from the exit pupil to the screen is at least 1.33 times larger than the diameter of the screen. Otherwise, a separate piece is needed for the Fresnel lens. Bradley et al, IEEE Trans. on Consumer Electronics; v. 31; (3); pp. 185-193; August 1985.
FIG. 3 shows a rear projection screen of a type called a TIR screen. Such a screen is described in U.S. Pat. No. 4,730,897, assigned to the present assignee, the entire contents of which are incorporated herein by reference. These screens use bulk diffusion, sometimes confined to a region 38 adjacent the lenticules, for vertical distribution and a single front surface 30 for the horizontal distribution. The shape of the lenticules 30a incorporates steep sidewalls 32 which totally internally reflect (TIRs) the light to the tip region 34 of the lenticules. The area in between the lenticules can be filled with a blackened substance 36 in a manner to maintain reflectivity of the steep sidewall surfaces, to thereby provide high contrast. This screen can have the Fresnel 39 on the back surface or on a second piece. This screen has similar characteristics to the double lenticular screen, i.e., high contrast and high gain, but the resolution can be increased because there is no relationship between screen thickness and resolution. Screens with 0.2 mm pitch have been made.
Rear projection screens typically contain some mechanism such as minute colloidal particles to diffuse the light into the desired viewing space. When these screens are used with high magnification systems in which the projection beam is nearly coherent, a disturbing artifact in the form of a speckle pattern is often observed. This speckle pattern is most pronounced in screens with high gain.
Speckle has also been observed in microfiche and microfilm readers where the f/# of the beam and the magnification is high.
Speckle is most often associated with laser illumination. See for example, D. Gabor, IBM J. Res. Develop., September 1970, pp 509-514. It appears when random surfaces are illuminated with nearly coherent beams.
Speckle reduction has been discussed in the literature. It is well known that to reduce the visibility of speckle, the coherence of the illumination beam must be destroyed. This has been achieved by moving one diffusing screen with respect to another and separating the diffusing surfaces. S. Lowenthal et al., J. Opt. Soc. Am., pp. 847-851 (1971) ; N. George et al., Opt. Commun., pp. 71-71 (1975); E. G. Rawson et al., J. Opt. Soc. Am., pp. 1290-1294 (1976) and L. G. Shirley et al., J. Opt. Soc. Am. A, pp 765-781 (1989).
We have observed that increasing the amount of diffusion and increasing the thickness of the diffuser also can reduce the visibility of the speckle, but on the other hand deteriorates the resolution o f the screen.